There are a number of different types of semiconductor-based imagers including charge coupled devices (CCD's), photodiode arrays, charge injection devices (CID's), hybrid focal plane arrays, and complementary metal oxide semiconductor (CMOS) imagers. Current applications of solid-state imagers include cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, automatic focus systems, star trackers, motion detector systems, image stabilization systems, and other image acquisition and processing systems.
Solid state imagers include an array of pixels that converts light energy received, through an optical lens, into electrical signals. Each pixel contains a photosensor for converting a respective portion of a received image into an electrical signal. The electrical signals produced by the array of photosensors are processed to render a digital image.
Imagers are sensitive to light in the visible spectrum. Naturally, however, the imagers used in digital imaging are essentially capturing black and white (light and dark) images. To capture color images, the spectral components of incident light must be separated and collected. Thus, to allow the photosensors to capture a color image, they must be able to separately detect photons traveling at different wavelengths. Accordingly, each pixel should be sensitive only to one color or spectral band. For this, a color filter array (CFA) is typically placed in front of the pixel array so that each pixel measures the light of the color of its associated filter. Color filters are typically pigmented or dyed material that will only allow a narrow band of visible light, e.g., red, blue, or green, to pass through. For most low cost CMOS or CCD imagers, the color filters are integrated with the pixel circuitry in a patterned array. A common example of a color filter pattern is the tiled color filter array illustrated in U.S. Pat. No. 3,971,065, and commonly referred to as “the Bayer pattern” color filter array, although other types of color filter array patterns exist. The use of a color filter array allows what would otherwise be black and white imagers to produce color images.
As shown in FIG. 1, the Bayer pattern 15 comprises an array of repeating red (R), green (G), and blue (B) filters. Half of the filters in the Bayer pattern 15 are green, while one quarter are red and the other quarter are blue. As shown, the Bayer pattern 15 repeats a row of alternating red and green color filters followed by a row of alternating blue and green filters.
The Bayer patterned color filter array (or filter array having another pattern) may be deposited/placed on top of an array 20 of pixels 22, as shown in cross section in FIG. 2. Specifically, an array 20 of pixels 22 is formed in a semiconductor substrate 10. Each of the pixels 22 has a photosensitive element 12, which may be any photon-to-charge converting device, such as a photogate, photoconductor or photodiode. The color filter array 15 may be formed over one or more metal layers 18 in the array 20, separated from the photosensor 12 by several insulating layers like an interlevel dielectric layer (ILD) 14 and a passivation layer 16. The metal layers 18 may be opaque and used to shield the area of the pixels that is not supposed to be light sensitive. Micro-lenses 21 are formed over the color filter array 15. In operation, incident light is focused by the micro-lenses 21 through the Bayer patterned color filter array 15 to the photosensitive elements 12.
One technique for fabricating color filter arrays uses evaporated colorants. To fabricate such color filter arrays over imagers, a photoresist containing a colorant, generally a pigment, is deposited on a semiconductor substrate. The pigmented photoresist is patterned, leaving resist with color pigment over the pixels in patterned areas. However, color pigment residue is left behind in the non-patterned areas after one set of color filter elements is patterned, which can interfere with formation of subsequent color filter array elements or degrade the overall imager performance. This color pigment residue is undesirable. One way in which this color pigment residue may be removed is with a descumming process, such as that disclosed U.S. Pat. No. 7,326,503, assigned to Micron Technology, Inc.
However, as shown in FIG. 3A, after a descumming process, there are many tiny particles containing pigment 25 which may remain on the surface of the color filter array 15. These tiny particles containing pigment 25 may affect the transmission performance of a micro-lens formed over the color filter array 15. Additionally, referring to FIG. 3B, these tiny particles containing pigment 25 are loose and if the next layer 35 is formed using a spin-on process, for example, the tiny particles containing pigment 25 can move and may combine with each other to form bigger clusters 30. This may result in defects or striation patterns that could cause the imager to fail.
Imagers also include light block structures surrounding the imaging pixels of the pixel array in order to cover the periphery circuits of an imager and/or so-called dark reference pixels which are not used for imaging. Additionally, these light block structures need to be absorptive so that no light flares that may affect the camera lens are present. One known method of forming these light block structures uses a black resist, such as, e.g., JSR black resist JSR BLACK812 or FFEM black resist SK5000L. One problem with using such black resists is the residue level which remains after formation. As shown in FIG. 6A, after light blocks 115 are formed, pigment residue 120 remains. Prior to FIG. 6A, the light blocks were patterned on the periphery area of the imager and/or pixel array as is known in the art. A plasma clean (or other acceptable process) is used to eliminate the pigment residues in the active array. However, tiny loose particles 145 can remain on the surface of the light blocks 115, as can be seen in FIG. 6B. These particles 145 may also form part of residue 25 over the color filter array 15 shown in FIG. 3A.
Accordingly, an improved method to reduce residue related defects of a color filter array or other structure formed of a color resist is desired.